A number of problems are presented when requiring an output signal to come up in a known state during power-up and remain in that state until power-up is complete. First, an external reset signal cannot be relied upon to bring the output signal up in a known state. The external reset signal may not become active until well after power-up has started. Furthermore, the external reset signal may not behave predictably during power-up. The second problem presented by such a requirement is that prior circuits cannot be depended upon to respond appropriately to the external reset signal during power-up, even if the external reset signal became active at the start of power-up and behaved predictably. Finally, an additional problem posed by such a requirement is that the completion of power-up cannot be accurately determined simply by observing Vcc.
Analog circuitry does not offer an acceptable solution to such a requirement because it must be tuned. Tuning implies stepping one or more production masks or reticles with the accompanying schedule delays.